What is food intolerance?
Food intolerance is the general term used to describe a range of reproducible adverse responses to a specific food or food ingredient. This general term includes allergic reactions that involve the immune system, adverse reactions resulting from enzyme deficiencies, pharmacological reactions and other defined responses.
Food intolerance does not include food poisoning due to bacteria and viruses, moulds, chemicals, toxins and irritants in foods nor does it include food aversions.
What is food allergy?
An allergic reaction to a food is an inappropriate reaction by the body’s immune system to the ingestion of a food that in the majority of individuals causes no adverse effects. Allergic reactions to foods vary in severity and may be fatal. In food allergy the immune system does not recognise as safe a protein component of the food to which the individual is sensitive. This component is termed the allergen. The immune system then typically produces immunoglobulin E (IgE) antibodies to the allergen, which trigger other cells to release substances that cause inflammation.
Allergic reactions are usually localised to a particular part of the body and the symptoms may include asthma, eczema, flushing, and swelling of tissues (such as the lips) or difficulty in breathing. A severe reaction may result in a rapid fall in blood pressure and severe shock.
What are the different types of allergy?
There are two well defined mechanisms through which allergic reactions to food can occur. Most cases of food allergy involve the production of antibodies known as immunoglobulin E (IgE) and are known as IgE mediated allergies. Symptoms develop quickly and can vary in severity.
The other recognised mechanism is a delayed response that may take hours or days to develop. These involve a different immune system component, T lymphocytes (T cells). The best example of this type of reaction is sensitivity to a protein, gluten, found in wheat and other cereals. Delayed reactions can also on occasion occur in response to a range of other foods, including milk and soya.
Which foods are the most common causes of allergic reactions?
The majority of allergic reactions to dietary components are caused by a small number of foods, namely:
- Cow’s milk
- Hen’s eggs
- Tree nuts
- Soya beans
- Soya products
- Shellfish, prawns
- Gluten containing cereals eg. Wheat
- Citrus fruits
In children, it has been estimated that nine out of ten reactions are to milk, soya, peanuts, tree nuts, or wheat gluten. Many of these reactions reduce after early childhood. A majority of allergic reactions in adults result from sensitisation to shellfish, fish, peanuts, and tree nuts. It is unusual for food allergy to begin in adulthood.
How does food allergy differ from other forms of food intolerance?
- Food intolerance reactions may vary the severity of the associated symptoms and the length of time for which they persist.
- For example peanut allergy is often life long and can cause severe, even life threatening reactions to tiny amounts of peanut protein.
- Cow’s milk intolerance may be severe in early life, but typically disappears as the child grows older.
- Similarly, egg intolerance is usually a temporary phenomenon associated with early childhood.
- Coeliac disease (gluten sensitivity) is normally life long and requires adherence to a diet that excludes all gluten. But in some people as the disease is mild and goes undiagnosed, the individual is not aware of any symptoms.
What causes lactose intolerance?
Lactose intolerance occurs in individuals who lack or have low levels of the enzyme lactase, which is needed to digest the sugar lactose (found in milk) to its constituent sugars (glucose and galactose) for absorption in the small intestines.
It is a condition seen in older children and adults. In the absence of lactase, undigested lactose passes into the large intestines causing diarrhoea, wind and general discomfort.
Complete avoidance is rarely necessary, as most people still produce some lactase enzyme. Yogurt is usually better tolerated than ordinary milk.
What is gluten sensitivity?
Coeliac disease is usually a life long condition requiring a strict gluten free diet, and the main organ affected is the small intestine. Ingestion of gluten activates immune cells in the small intestine, which trigger inflammation and local damage. This disrupts the normal processes of food digestion and absorption. As a result, untreated coeliac patients lose weight, develop deficiencies such as anaemia, and experience symptoms such as diarrhoea.
Gluten is found in wheat, barley and rye, which means that many dietary staples such as bread, many breakfast cereals and foods like pizza and pasta can no longer be eaten.
What is peanut allergy?
As peanut allergy is one of the few allergies that is typically life long. Its prevalence is estimated to be greater in adults than in children. It remains unclear whether the sensitisation to peanuts is inherited or not. There is no justification in the avoidance of peanuts if there is no history of allergy in parents or brothers and sisters of the new baby.
Is migraine caused by food allergy?
It is likely that some of the headaches and migraines experienced by some people are provoked by food. However it is unlikely to be a single food that is a common cause. Coffee, chocolate, and alcoholic drinks are possible triggers in some people.